A prior art machine for plasma treatment of substrates of permeable material comprises a cylindrical hollow drum. The drum can rotate about its own central axis of rotation to convey the substrate to be processed. In particular, during machine operation the substrate partly covers an outer cylindrical surface of the drum.
The machine comprises a first electrode covering the outer cylindrical surface of the drum. In more detail, during machine operation the first electrode is interposed between the drum and the substrate.
The machine also comprises a second electrode facing the first and outside the drum. During machine operation the substrate to be processed is positioned between the first and second electrodes. In other words, the second electrode partly surrounds the drum.
The second electrode comprises a plurality of cylindrical bars each with its own main axis of extension parallel with the central axis of rotation of the drum. The bars are equidistant from the central axis of rotation of the drum. They are also spaced at equal angles to each other.
The cylindrical outer surface of the drum has openings allowing the passage of gas from the outside towards an inner cavity in the drum. The inner cavity is connected to a suction fan. During operation the suction fan forces the gas to move from the outside to the inside of the drum, thus creating a flow of gas between the bars of the second electrode and the first electrode. In particular, during operation said flow of gas passes through the substrate placed between the electrodes.
During operation there is a difference in electric potential across the two electrodes, which generates electric discharges between the bars of the second electrode and the first electrode. In this way, the gas travelling between the first and second electrodes is ionised, producing the plasma close to the substrate and allowing the substrate to be treated.
The prior art machine for plasma treatment of substrates of permeable material has significant disadvantages. First, the rotation of the drum subjects the substrate to tangential forces able to deform and pull apart low resistance materials (for example bands of fibres).
Another disadvantage is the different distances between substrate and the first and second electrodes. That results in uneven treatment on two faces of the substrate.